http://www.25-seven.com/257/publications/25-Seven_Tech_Paper_Arbitron_PPM.pdf
Arbitron is a company that tracks ratings, and has started watermarking broadcast music using a device inline at the source. They claim it is "inaudible" but the PDF above shows that even radio listeners could hear it and it needs tweaking. The only way this would work is if all broadcasters use the technology, which seems to be Arbitron (Lockheed Martin)'s intent.
It would be interesting to hear what watermarked music sounds like and if we can hear the noise.
